Q-Post is probably one of the cheapest. in arabic they call this service as Mumtaz Post and commonly known as EMS.
as your laptop is 3kg so the charge is gonna be 140 QR, and it takes maximum 4 days.

Tel. (+974) 4627776, 4627777, 4627778, 4627779
Address: Al Tyseer, Airport street No. 42, Opposite of Doha International Airport

web ... http://www.qatarposts.com/gpc/ems.htm

hope it helps !